While I prefer to wrangle and manage the 250-305 twins, I do have experience with the various four cylinder bikes of the 1970-80s. My high school buddy, Art, picked up this bike a few months back and brought it by for a quick evaluation. With only 11k original miles showing on the speedometer and the presence of the ubiquitous Vetter Windjammer fairing installed, it spoke touring bike all over.

Unfortunately it had a bad misfire on the #1 cylinder and watching/listening to it run and cough, I observed fumes exiting the rubber intake manifold, even though the clamps were as tight as they could go. NOT a good sign... I richened up the idle mixture screw a bit and that seemed to settle it down, somewhat. I encouraged him to leave it with me so I could replace the intake manifold rubbers and also address other issues, including oil leaks all over the top end and the reason that black electrical tape was covering the oil warning light on the dash board cluster! I started to order parts, but it was almost another month before the bike came to me for repairs.

Replacement parts came in from Honda warehouses, as well as a speedy, eBay seller in Japan who shipped a top-end gasket set to me in four days! The likely cause of the oil warning light was the sending unit, mounted atop the oil pump, which is located beneath the left side engine cover, to the rear of the alternator. Speaking of alternators… These late model CB650s had inherited the charging systems from the 1979-83 DOHC CB750/900/1000/1100 fours, all of which are plagued with charging system issues, mostly related to defective rotors. Worn carbon brushes are also a cause for charging system failures.

After finally dropping the bike off to me, I ran my buddy Art back home and then returned to start the teardown process. While my specialty is the 250-305s, I have worked on several CB500-550F models through the years, but never a CB650, which is a derivative of the earlier 500-550F models. There were some major changes involved to bring the CB650 up to that displacement size and also to reinforce the various parts for increased horsepower and to meet emission requirements of the 1980s. The biggest change was the cylinder head cover, which formerly housed 8 little tappet covers to access the valve adjusters directly at the valve stems. The new cover encompassed the whole cylinder head, valve train and raised the whole height up another inch or so. In order to access the valve adjusters, there are three separate top cover plates, now, which allow access to the valve adjuster nuts/screws. The actual measurement of the valve lash is done on the rocker end where it contacts the camshaft lobe instead of the valve stem end.

The camchain, which had been a single-row 219 pitch chain, was ditched for a Morse Hy-Vo multi-plate chain which has much more strength and tend to run quieter than the old roller chains. The original top end fasteners were a series of long studs, topped off by either flat washer/nuts or inside the engine a few cap-nuts and some large half-dollar sized rubber plugs which sealed off the nuts down below the oil reservoirs in the cylinder head. This system is similar to the old SOHC CB750 motors, which also use the flat rubber sealing plugs to prevent oil leaks. This particular bike only had 11,000 miles showing on the odometer, but it was oozing oil out all over the place, along the sides and front of the cylinder head and rocker arm cover gaskets.

The carburetors seemed to be leaking fuel from either dirty float valves and/or the interconnecting fuel tube and accelerator pump tube o-rings. The accelerator pump diaphragms can fail causing leaks from the pump cover areas. There were some dark stains at the center of the carb rack on the fuel connector tube, so it looks like the carb rack will have to be disassembled far enough to get to the o-rings at the ends of each of the tubing sections. These carburetors are CV-style with metal slides, related to the DOHC fours of that era, replacing the earlier ganged-together slide type carburetors, which did not have accelerator pump features.

The top end teardown is pretty straightforward, much like the earlier models, however instead of long studs coming up from the crankcases, the whole top end is held together with LONG flanged head bolts. Before you get down to the cylinder head bolt hexes, you have to remove four 10mm nuts/washers, then a whole host of little 8mm head short bolts, which clamp the rocker arm cover down to the cylinder head. All the small hold-down bolts can be loosened up and then left in the rocker arm cover, which is removed as a unit from the cylinder head.

One of the forward, inner cylinder head bolts is open to incoming air, water and dirt flung up from the front wheel, was challenging to remove as the dirt was packed around the bolt head and kept the socket from grabbing the bolt head firmly. There are two small 6mm (10mm head) bolts at the front and rear of the cylinder head, right in the center, which must be removed, but can be overlooked if they are covered with grease and dirt, like these were. Once the final fasteners were removed, the cylinder head lifted off easily, leaving the telltale signs of where oil had been oozing out various gaskets and mating surfaces. It appears that the tachometer drive gear seal was leaking, along with the housing’s o-ring, which caused oil to drool down the front of the motor and the run along the fins and cylinder block surfaces.

With the cylinder head removed, the next steps are cleaning off the old oil/grease/dirt and then removal of all of the valves to check their seats and to replace their 25-year old valve stem seals. Valve stem seal hardening is a real problem now, as these tiny rubber parts have aged and been subjected to years of engine heat and various chemicals in oil. When engines exhibit oil smoking when coasting down long hills and when taking off after a prolonged stop at idle, then it is a pretty good bet that the engine needs stem seals, not piston rings.

Day 2: The cylinder head was cleaned and valves removed for valve stem removal and cleaning of any old deposits. And were there some DEPOSITS! You probably have seen automotive ads where they claim to clean the intake valve deposits using specially formulated gasoline, additives or some kind of injectable chemicals to clean the back side of the intake valve. Well, I can’t imagine what else, other than the wire brush for sixty seconds, could have cleaned the build-up on these intake valves! Black carbon deposits were thick and scaly, a combination of leaking stem seals and/or poor gasoline detergents. All four valves looked about the same and took the same amount of time to clean on the wire wheel. Once the valves were removed, new valve stem seals were installed and the cleaned-up valves replaced. All of the valve seats seemed pretty well defined; however there were some tiny signs of pitting on the exhaust valve seats.

Having scrubbed up the top end parts as well as I can with some relatively non-toxic chemicals, I took the assembled head to fit back on the engine. Removing the head gasket from the aftermarket brand D&K kit lead to a bit of shock as the holes were not all that well placed in the new head gasket, causing the gasket to buckle up in the middle and the oil feed holes, which are sealed with rubber sealing rings were overlapping the edges of the seals. Using an 18v drill and a stepped drill bit, I carefully massaged the holes open here and there, working the inaccuracies down to where I hoped they would not cause an issue with oil flow and/or leaks.

There were problems getting the head bolts to screw down into the crankcase due to tiny rock debris which fell into the bolt holes as the bolts were withdrawn the first time. A combination of a long scribe, some spray brake cleaner and compressed air finally cleared the holes and allowed for bolt insertion.

With the cylinder head finally torque down, the camshaft and sprocket were next on the installation list. The thick Hy-Vo chain is a little more difficult to work over the camsprocket then work the sprocket over the end of the camshaft as it dropped down into the cam bearings. With the 1-4 pistons all the way at TDC, the camchain had to be fed over the sprocket a few teeth at a time until the index marks showed up evenly on both sides of the sprocket. These are just a pair of punch marks that wind up horizontally placed even with the cylinder head, when the cam timing is correct. With cam timing set the previously retracted camchain tensioner nut at the base of the cylinders was loosened up and the tensioner smacked up against the camchain with a smart thud. Retighten the nut, once again and it is all set for operation.

With the wriggly o-ring under control, the cover dropped onto the camshaft and cylinder head with care given to making sure the rocker arms were all loose at the adjusters. A few of the cam lobes are going to be sticking up, putting pressure on the camshaft and cover, but a gentle tightening of all the cover bolts, a little at a time, eased it all into place. The next step was to set the valve lash, using long feeler gauges fitted between the rocker arm pad and the camshaft lobe. The intakes are .002” and the exhausts are set at .003”. You can set half of the valves on one 1-4 TDC position and then the other half when the engine is turned one revolution further.

The tappet covers are next, again with new rubber gasket packings that were just a little off the mark but finally worked themselves into place. The top rocker arm cover uses a specially designed o-ring to seal the cover against the cylinder head. This formed piece seemed to be a few mm short of the original designed part, which caused it to creep out of the channel provided for the o-ring to be retained. With some stretching and holding various corners down, it finally appeared to be ready to stay put and allow installation of the cover.

With the top end together, the intake manifolds and carburetor bank were the next items on the agenda. The carburetors proved to be a real PIA for several reasons; including fuel leaks on the fuel feed and accelerator pump tubings. In order to service the o-rings the carburetors have to be disassembled from the brackets, then the choke plate retainer screws removed in order to split the carburetors apart from each other. Working on these carburetors rates up there with watch repairs for a couple of hours.

When the carb rack was reassembled and the bike initially fired up, the carburetors went into a full flood overflow, drowning the engine with raw fuel. This condition is generally caused by blocked bowl vents which help equalize the outside atmospheric pressure with the changing pressures inside the carburetor circuits. Blowing backwards through the ganged-together vent fitting hoses, appeared to show the passages open, but putting the rack back on again yielded the same results. A third (or fourth?) removal included extended checking of the venting system and even some Internet checking for similar issues reported from other CB650 owners; and there were more than a few who experienced the same condition. After cleaning and checking everything imaginable once again, the rack was mounted back up (on new manifolds, lubricated with silicone grease) and all of a sudden they settled down and behaved normally. Accessing the push-pull throttle cable ends and the throttle cable connections is nearly impossible if you don’t know how they are fitted to the throttle fittings. Even when you do know how they go together, there is precious little room to fish the cable ends on and off the carburetor rack… extreeeemly frustrating!

Eventually the bike gave up fighting me before I gave up working on it and a successful test ride ensued. Riding at freeway speeds gives the rider an impression that the bike needs another couple of gears to get comfortable on the highway. At 70mph, the tach is up around 6k rpms and the motor is really sounding busy. Even though the redline is up around 9,500 rpms and you are only 2/3rds the way up the scale, the engine sounds like it is really working hard. If it were mine I would definitely put another tooth on the front sprocket or something smaller on the rear sprocket to lower the rpms. In the end, the repairs appeared to be successful with no apparent oil leaks after the first test ride and the engine was making some good power when you cracked the throttle open. It is interesting to go back in time to revisit the 1980s model bikes, but the addition of emission control features and the tiny spaces where everything is packed together is so very difficult for mechanics to work on without cursing each and every step along the way. It might be prudent to call a priest and have them cast out whatever demons are left with that bike, in addition to the curses placed upon it out of sheer frustration and difficulty. I recall that the introduction of the updated CB650s left a lot of magazine testers amazed at the transformation which took place to create a powerful machine out of the original CB500F model, but obviously they didn’t have to do any service work on them! Working on this bike has certainly cured me of wanting to add a CB650 to my collection or even as a daily driver. The electronic ignition system is certainly an improvement over the old points and condenser ignitions, but the brush-type alternator and the complex CV carburetors used to modernize this model have proven to be troublesome and challenging to maintain over time.

Still to be resolved is the case of the defectively made gasket sets… to be continued.

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Bill "MrHonda" Silver, is a San Diego native, with over 40 years of Honda motorcycle ownership. Bill has written about vintage Hondas for the VJMC newsletter (as newsletter editor for two years), was published at MBI (Motorbooks, Intl) in 2000 with his comprehensive Classic Honda Motorcycle Buyer's Guide book (due to go into reprinting in 2011) and is the foremost expert in the 1960s Honda 250-305cc twin series models.
Bill volunteers his time with AllExperts.com site, where he has answered over 3900 questions about Honda (and a few other makes) motorcycle repair. Contact Bill through his website.